Laser beam machining method

ABSTRACT

A laser processing method which can accurately cut an object to be processed along a line to cut is provided. A modified region  7  formed by multiphoton absorption forms a cutting start region  8  within an object to be processed  1  along a line  5  along which the object is intended to be cut. Thereafter, the object  1  is irradiated with laser light L 2  transmittable through an unmodified region of the object  1 , so as to generate fractures  24  from the cutting start region  8  acting as a start point, whereby the object  1  can accurately be cut along the line  5  along which the object is intended to be cut. Expanding an expandable film  19  having the object  1  secured thereto separates individual chips  25  from each other, which can further improve the reliability in cutting the object  1  along the line  5  along which the object is intended to be cut.

TECHNICAL FIELD

The present invention relates to a laser processing method used forcutting an object to be processed such as semiconductor materialsubstrate, piezoelectric material substrate, or glass substrate.

BACKGROUND ART

An example of literatures disclosing a conventional technique of thiskind is International Publication Pamphlet No. 02/22301. Thespecification of this literature discloses a technique of irradiating anobject to be processed with laser light, so as to form a modified regionwithin the object along a line along which the object is intended to becut, and cutting the object from the modified region acting as a startpoint.

DISCLOSURE OF THE INVENTION

Since the technique disclosed in the above-mentioned literature is aquite effective technique which can accurately cut the object along aline along which the object is intended to be cut, there has been ademand for a technique which can cut the object from the modified regionmore accurately.

In view of such circumstances, it is an object of the present inventionto provide a laser processing method which can accurately cut the objectfrom the line along which the object is intended to be cut.

In order to achieve the above-mentioned object, in one aspect, thepresent invention provides a laser processing method comprising a firststep of irradiating a wafer-like object to be processed with laser lightwhile locating a light-converging point within the object, so as to forma modified region due to multiphoton absorption within the object, andcausing the modified region to form a cutting start region inside of alaser light entrance surface of the object by a predetermined distancealong a line along which the object is intended to be cut in the object;and a second step of irradiating the modified region with laser lighttransmittable through an unmodified region of the object and moreabsorbable by the modified region than by the unmodified region afterthe first step, so as to generate a stress at a portion where the objectis cut along the line along which the object is intended to be cut.

This laser processing method forms a modified region within an object tobe processed by irradiating the object with laser light while locating alight-converging point within the object and utilizing a phenomenon ofmultiphoton absorption in the first step. When a start point exists at aportion where the object is to be cut, the object can be cleaved with arelatively small force, so as to be cut. This laser processing methodirradiates the object with laser light transmittable through anunmodified region of the object and more absorbable by the modifiedregion than by the unmodified region along a line along which the objectis intended to be cut in the second step, so that the object is heatedalong the modified region, whereby a stress such as thermal stress dueto a temperature difference occurs. This stress grows a crack in thethickness direction of the object from the modified region acting as astart point, thereby making it possible to cleave and cut the object.Thus, the object can be cut by a relatively small force such as a stresstypified by a thermal stress due to a temperature difference, wherebythe object can be cut with a high accuracy without generatingunnecessary fractures deviating from the line along which the object isintended to be cut on a surface of the object.

This laser processing method forms the modified region by locallygenerating multiphoton absorption within the object in the first step.In the second step, laser light transmittable through the unmodifiedregion is emitted. Therefore, the laser light is hardly absorbed at thesurface of the object, so that the surface of the object is hardlymolten in both steps. The unmodified region refers to a region notformed with a modified region after the first step. The light-convergingpoint refers to a portion where the laser light is converged. The linealong which the object is intended to be cut may be a line actuallydrawn on the surface of the object or therewithin, or a virtual line.

In another aspect, the present invention provides a laser processingmethod comprising a first step of irradiating a wafer-like object to beprocessed with laser light while locating a light-converging pointwithin the object under a condition with a peak power density of atleast 1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1μs or less, so as to form a modified region including a crack regionwithin the object, and causing the modified region to form a cuttingstart region inside of a laser light entrance surface of the object by apredetermined distance along a line along which the object is intendedto be cut in the object; and a second step of irradiating the modifiedregion with laser light transmittable through an unmodified region ofthe object and more absorbable by the modified region than by theunmodified region after the first step, so as to generate a stress at aportion where the object is cut along the line along which the object isintended to be cut.

This laser processing method irradiates the object with laser lightwhile locating a light-converging point within the object under acondition with a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 μs or less in the firststep. Consequently, a phenomenon of optical damage due to multiphotonabsorption occurs within the object. This optical damage induces athermal distortion within the object, thereby forming a crack regiontherewithin. Since this crack region is an example of theabove-mentioned modified region while the second step is equivalent tothat mentioned above, this laser processing method enables laserprocessing without melting the surface of the object or generatingunnecessary fractures thereon deviating from the line along which theobject is intended to be cut. An example of the object to be processedin this laser processing method is a member including glass. The peakpower density refers to the electric field intensity at thelight-converging point of pulsed laser light.

In still another aspect, the present invention provides a laserprocessing method comprising a first step of irradiating a wafer-likeobject to be processed with laser light while locating alight-converging point within the object under a condition with a peakpower density of at least 1×10⁸ (W/cm²) at the light-converging pointand a pulse width of 1 μs or less, so as to form a modified regionincluding a molten processed region within the object, and causing themodified region to form a cutting start region inside of a laser lightentrance surface of the object by a predetermined distance along a linealong which the object is intended to be cut in the object; and a secondstep of irradiating the modified region with laser light transmittablethrough an unmodified region of the object and more absorbable by themodified region than by the unmodified region after the first step, soas to generate a stress at a portion where the object is cut along theline along which the object is intended to be cut.

This laser processing method irradiates the object with laser lightwhile locating a light-converging point within the object under acondition with a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 μs or less in the firststep. Consequently, the inside of the object is locally heated bymultiphoton absorption. The heating forms a molten processed regionwithin the object. Since this molten processed region is an example ofthe above-mentioned modified region while the second step is equivalentto that mentioned above, this laser processing method enables laserprocessing without melting the surface of the object or generatingunnecessary fractures thereon deviating from the line along which theobject is intended to be cut. An example of the object to be processedin this laser processing method is a member including a semiconductormaterial.

In still another aspect, the present invention provides a laserprocessing method comprising a first step of irradiating a wafer-likeobject to be processed with laser light while locating alight-converging point within the object under a condition with a peakpower density of at least 1×10⁸ (W/cm²) at the light-converging pointand a pulse width of 1 ns or less, so as to form a modified regionincluding a refractive index change region as a region with a changedrefractive index within the object, and causing the modified region toform a cutting start region inside of a laser light entrance surface ofthe object by a predetermined distance along a line along which theobject is intended to be cut in the object; and a second step ofirradiating the modified region with laser light transmittable throughan unmodified region of the object and more absorbable by the modifiedregion than by the unmodified region after the first step, so as togenerate a stress at a portion where the object is cut along the linealong which the object is intended to be cut.

This laser processing method irradiates the object with laser lightwhile locating a light-converging point within the object under acondition with a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 ns or less. Whenmultiphoton absorption is thus generated within the object with a veryshort pulse width, the energy due to the multiphoton absorption is notconverted into thermal energy, whereas an eternal structural change suchas ionic valence change, crystallization, or polarization orientation isinduced within the object, whereby a refractive index change region isformed. Since the refractive index change region is an example of theabove-mentioned modified region while the second step is equivalent tothat mentioned above, this laser processing method enables laserprocessing without melting the surface of the object or generatingunnecessary fractures thereon deviating from the line along which theobject is intended to be cut. An example of the object to be processedin this laser processing method is a member including glass.

Preferably, the second step performs the same laser light irradiation aswith the first step while locating a light-converging point at themodified region. Even when the second step performs the same laser lightirradiation as with the first step, the laser light absorption due toscattering by the modified region, changes in physical properties of themodified region, etc. or the occurrence of multiphoton absorption in themodified region can heat the object along the modified region withoutmelting the front face of the object, thereby generating a stress suchas thermal stress due to a temperature difference.

In still another aspect, the present invention provides a laserprocessing method comprising the steps of irradiating a wafer-likeobject to be processed secured to a surface of an expandable holdingmember with laser light while locating a light-converging point withinthe object, so as to form a modified region within the object, andcausing the modified region to form a cutting start region inside of alaser light entrance surface of the object by a predetermined distancealong a line along which the object is intended to be cut in the object;irradiating the modified region with laser light transmittable throughan unmodified region of the object after the step of forming the cuttingstart region, so as to cut the object along the line along which theobject is intended to be cut; and expanding the holding member after thestep of cutting the object, so as to separate cut portions of the objectfrom each other.

In this laser processing method, the modified region formed bymultiphoton absorption can form a cutting start region within the objectalong a desirable line along which the object is intended to be cut theobject. Then, irradiating the object with laser light transmittablethrough the unmodified region (the part of the object other than themodified region) along the line along which the object is intended to becut can generate fractures from the cutting start region acting as astart point, whereby the object can be cut accurately along the linealong which the object is intended to be cut. Expanding the holdingmember having the object secured thereto separates portions of theobject from each other, whereby the reliability in cutting the objectalong the line along which the object is intended to be cut can furtherbe improved.

In still another aspect, the present invention provides a laserprocessing method comprising the steps of irradiating a wafer-likeobject to be processed secured to a surface of an expandable holdingmember with laser light while locating a light-converging point withinthe object, so as to form a modified region within the object, andcausing the modified region to form a cutting start region inside of alaser light entrance surface of the object by a predetermined distancealong a line along which the object is intended to be cut in the object;irradiating the modified region with laser light transmittable throughan unmodified region of the object after the step of forming the cuttingstart region; and expanding the holding member after the step ofirradiating the object, so as to cut the object and separate cutportions of the object from each other.

This laser processing method can form a cutting start region within theobject along a line along which the object is intended to be cut as withthe laser processing methods mentioned above. Then, irradiating theobject with laser light transmittable through the unmodified regionalong a line along which the object is intended to be cut allowsfractures started from the cutting start region to reach the front andrear faces of the object with a smaller force than that in the casewithout such irradiation. Therefore, the holding member having theobject secured thereto can be expanded with a smaller force, so that theobject can be cut accurately. Expanding the holding member separatesportions of the object from each other, so that the reliability incutting the object along the line along which the object is intended tobe cut can further be improved.

The cutting start region refers to a region to become a start point forcutting when cutting the object. Therefore, the cutting start region isa part to cut where cutting is to be done in the object. The cuttingstart region may be made by a modified region formed continuously ormodified regions formed intermittently. The object may be formed from asemiconductor material, in which the modified region is a moltenprocessed region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an object to be processed during laserprocessing by the laser processing method in accordance with anembodiment;

FIG. 2 is a sectional view of the object taken along the line II-II ofFIG. 1;

FIG. 3 is a plan view of the object after the laser processing by thelaser processing method in accordance with the embodiment;

FIG. 4 is a sectional view of the object taken along the line IV-IV ofFIG. 3;

FIG. 5 is a sectional view of the object taken along the line V-V ofFIG. 3;

FIG. 6 is a plan view of the object cut by the laser processing methodin accordance with the embodiment;

FIG. 7 is a graph showing relationships between the field intensity andcrack spot size in the laser processing method in accordance with theembodiment;

FIG. 8 is a sectional view of the object in a first step of the laserprocessing method in accordance with the embodiment;

FIG. 9 is a sectional view of the object in a second step of the laserprocessing method in accordance with the embodiment;

FIG. 10 is a sectional view of the object in a third step of the laserprocessing method in accordance with the embodiment;

FIG. 11 is a sectional view of the object in a fourth step of the laserprocessing method in accordance with the embodiment;

FIG. 12 is a view showing a photograph of a cross section of a part of asilicon wafer cut by the laser processing method in accordance with theembodiment;

FIG. 13 is a graph showing relationships between the laser lightwavelength and the transmittance within the silicon substrate in thelaser processing method in accordance with the embodiment;

FIG. 14 is a schematic diagram showing the laser processing apparatus inaccordance with Example 1;

FIG. 15 is a flowchart for explaining the laser processing method inaccordance with Example 1;

FIG. 16 is a sectional view of the object including a crack regionduring processing in a modified region forming step in accordance withExample 1;

FIG. 17 is a sectional view of the object including the crack regionduring laser processing in a stress step in accordance with Example 1;

FIG. 18 is a plan view of the object for explaining a pattern which canbe cut by the laser processing method in accordance with Example 1;

FIG. 19 is a plan view of the object in accordance with Example 2;

FIG. 20 is a sectional view showing a state where a cutting start regionis formed in the object in accordance with Example 2;

FIG. 21 is a sectional view showing a state where the object inaccordance with Example 2 is irradiated with laser light transmittablethrough an unmodified region of the object and more absorbable by themodified region than by the unmodified region;

FIG. 22 is a sectional view showing a state where the object inaccordance with Example 2 is set to a film expanding apparatus;

FIG. 23 is a sectional view showing a state where an expandable filmhaving the object in accordance with Example 2 secured thereto isexpanded; and

FIG. 24 is a sectional view showing a state where the object inaccordance with Example 3 is irradiated with laser light transmittablethrough the unmodified region of the object and more absorbable by themodified region than by the unmodified region.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained in detail with reference to the drawings. The laserprocessing method in accordance with this embodiment forms a modifiedregion by utilizing multiphoton absorption. The multiphoton absorptionis a phenomenon occurring when the intensity of laser light is veryhigh. Therefore, the multiphoton absorption will be explained first inbrief.

A material becomes transparent when its absorption bandgap E_(G) isgreater than photon energy hν. Hence, a condition under which absorptionoccurs in the material is hν>E_(G). However, even when opticallytransparent, the material generates absorption under a condition ofnhν>E_(G) (where n=2, 3, 4, . . . ) if the intensity of laser lightbecomes very high. This phenomenon is known as multiphoton absorption.In the case of pulsed waves, the intensity of laser light is determinedby the peak power density (W/cm²) of laser light at a light-convergingpoint. The multiphoton absorption occurs under a condition where thepeak power density is 1×10⁸ (W/cm²) or greater, for example. The peakpower density is determined by (energy of laser light at thelight-converging point per pulse)/(beam spot cross-sectional area oflaser light×pulse width). In the case of continuous waves, the intensityof laser light is determined by the field intensity (W/cm²) of laserlight at the light-converging point.

The principle of the laser processing method in accordance with theembodiment using such multiphoton absorption will be explained withreference to FIGS. 1 to 6. FIG. 1 is a plan view of an object to beprocessed 1 during laser processing. FIG. 2 is a sectional view of theobject 1 taken along the line II-II of FIG. 1. FIG. 3 is a plan view ofthe object 1 after the laser processing. FIG. 4 is a sectional view ofthe object 1 taken along the line IV-IV of FIG. 3. FIG. 5 is a sectionalview of the object 1 taken along the line V-V of FIG. 3. FIG. 6 is aplan view of the cut object 1.

As shown in FIGS. 1 and 2, a line 5 along which the object is intendedto be cut exists on a front face 3 of the object 1. The line 5 alongwhich the object is intended to be cut is a virtual line extendingstraight. The laser processing in accordance with this embodimentirradiates the object 1 with laser light L while locating alight-converging point P within the object 1 under a conditiongenerating multiphoton absorption, so as to form a modified region 7.The light-converging point P is a position at which the laser light L isconverged.

The laser light L is relatively moved along the line 5 along which theobject is intended to be cut (i.e., in the direction of arrow A), so asto shift the light-converging point P along the line 5 along which theobject is intended to be cut. Consequently, as shown in FIGS. 3 to 5,the modified region 7 is formed along the line 5 along which the objectis intended to be cut only within the object 1. In the laser processingmethod in accordance with this embodiment, the modified region 7 is notformed by the heat generated from the object 1 absorbing the laser lightL. The laser light L is transmitted through the object 1, so as togenerate multiphoton absorption therewithin, thereby forming themodified region 7. Therefore, the front face 3 of the object 1 hardlyabsorbs the laser light L and does not melt.

When a start point exists in a portion to cut at the time of cutting theobject 1, the object 1 fractures from the start point, whereby theobject 1 can be cut with a relatively small force as shown in FIG. 6.Therefore, the object 1 can be cut without generating unnecessaryfractures on the front face 3 of the object 1.

There seem to be the following two ways of cutting an object to beprocessed from the modified region acting as the start point. The firstcase is where an artificial force is applied to the object after formingthe modified region, so that the object fractures from the modifiedregion, whereby the object is cut. This is the cutting in the case wherethe object has a large thickness, for example. Applying an artificialforce refers to exerting a bending stress or shear stress to the objectalong the cutting start region, or generating a thermal stress byapplying a temperature difference to the object, for example. The othercase is where the forming of the modified region causes the object tofracture naturally in its cross-sectional direction (thicknessdirection) from the modified region acting as a start point, therebycutting the object. This becomes possible, for example, if one modifiedregion is formed when the object 1 has a small thickness, or if aplurality of modified regions are formed in the thickness direction whenthe object has a large thickness. Even in this naturally fracturingcase, fractures do not extend onto the front face at a portioncorresponding to an area not formed with the modified region, so thatonly the portion corresponding to the area formed with the modifiedregion can be cleaved, whereby cleavage can be controlled well. Such acleaving method with a favorable controllability is quite effective,since the object such as silicon wafer has recently been apt to decreaseits thickness.

The modified region formed by multiphoton absorption in this embodimentencompasses the following cases (1) to (3):

(1) Case where the Modified Region is a Crack Region Including One Crackor a Plurality of Cracks

An object to be processed (e.g., glass or a piezoelectric material madeof LiTaO₃) is irradiated with laser light while locating alight-converging point therewithin under a condition with a fieldintensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less. This magnitude of pulse width is acondition under which a crack region can be formed only within theobject while generating multiphoton absorption without causingunnecessary damages to the object. This generates a phenomenon ofoptical damage by multiphoton absorption within the object. This opticaldamage induces a thermal distortion within the object, thereby forming acrack region therewithin. The upper limit of field intensity is 1×10¹²(W/cm²), for example. The pulse width is preferably 1 ns to 200 ns, forexample. The forming of a crack region by multiphoton absorption isdisclosed, for example, in “Internal Marking of Glass Substrate withSolid-state Laser Harmonics”, Proceedings of the 45th Laser MaterialsProcessing Conference (December, 1998), pp. 23-28.

The inventors determined the relationship between field intensity andcrack size by an experiment. The following are conditions of theexperiment.

(A) Object to be processed: Pyrex (registered trademark) glass (with athickness of 700 μm and an outer diameter of 4 inches)

(B) Laser

-   -   light source: semiconductor laser pumping Nd:YAG laser    -   wavelength: 1064 nm    -   laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   oscillation mode: Q-switched pulse    -   repetition frequency: 100 kHz    -   pulse width: 30 ns    -   output: output<1 mJ/pulse    -   laser light quality: TEM₀₀    -   polarizing property: linear polarization

(C) Condenser lens

-   -   transmittance at a laser light wavelength: 60%

(D) Moving rate of the mounting table mounting the object: 100 mm/sec

The laser light quality of TEM₀₀ means that the light-convergingcharacteristic is so high that convergence to about the wavelength oflaser light is possible.

FIG. 7 is a graph showing the results of the above-mentioned experiment.The abscissa indicates the peak power density. Since the laser light ispulsed laser light, the field intensity is represented by the peak powerdensity. The ordinate indicates the size of a crack spot formed withinthe object by one pulse of laser light. The crack spot size is the sizeof a part yielding the maximum length among forms of crack spots. Datarepresented by black circles in the graph refer to a case where thecondenser lens (C) has a magnification of ×100 and a numerical aperture(NA) of 0.80. On the other hand, data represented by whitened circles inthe graph refer to a case where the condenser lens (C) has amagnification of ×50 and a numerical aperture (NA) of 0.55. Crack spotsare seen to occur within the object from when the peak power density isabout 10¹¹ (W/cm²) and become greater as the peak power densityincreases.

A mechanism by which the objet to be processed is cut by forming a crackregion will now be explained with reference to FIGS. 8 to 11. As shownin FIG. 8, the object 1 is irradiated with laser light L while thelight-converging point P is located within the object 1 under acondition where multiphoton absorption occurs, so as to form a crackregion 9 therewithin along a line to cut. The crack region 9 is a regioncontaining one crack or a plurality of cracks. A crack further growsfrom the crack region 9 acting as a start point as shown in FIG. 9, andreaches the front face 3 and rear face 21 of the object 1 as shown inFIG. 10, whereby the object 1 fractures and is consequently cut as shownin FIG. 11. The crack reaching the front and rear faces of the objectmay grow naturally or as a force is applied to the object.

(2) Case where the Modified Region is a Molten Processed Region

An object to be processed (e.g., semiconductor material such as silicon)is irradiated with laser light while locating a light-converging pointwithin the object under a condition with a field intensity of at least1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1 μs orless. As a consequence, the inside of the object is locally heated bymultiphoton absorption. This heating forms a molten processed regionwithin the object. The molten processed region encompasses regions oncemolten and then re-solidified, regions just in a molten state, andregions in the process of being re-solidified from the molten state, andcan also be referred to as a region whose phase has changed or a regionwhose crystal structure has changed. The molten processed region mayalso be referred to as a region in which a certain structure has changedto another structure among monocrystal, amorphous, and polycrystalstructures. For example, it means a region having changed from themonocrystal structure to the amorphous structure, a region havingchanged from the monocrystal structure to the polycrystal structure, ora region having changed from the monocrystal structure to a structurecontaining amorphous and polycrystal structures. When the object to beprocessed is of a silicon monocrystal structure, the molten processedregion is an amorphous silicon structure, for example. The upper limitof field intensity is 1×10¹² (W/cm²), for example. The pulse width ispreferably 1 ns to 200 ns, for example.

By an experiment, the inventors verified that a molten processed regionwas formed within a silicon wafer. The following are conditions of theexperiment.

(A) Object to be processed: silicon wafer (with a thickness of 350 μmand an outer diameter of 4 inches)

(B) Laser

-   -   light source: semiconductor laser pumping Nd:YAG laser    -   wavelength: 1064 nm    -   laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   oscillation mode: Q-switched pulse    -   repetition frequency: 100 kHz    -   pulse width: 30 ns    -   output: 20 μJ/pulse    -   laser light quality: TEM₀₀    -   polarizing property: linear polarization

(C) Condenser lens

-   -   magnification: ×50    -   N.A.: 0.55    -   transmittance at a laser light wavelength: 60%

(D) Moving rate of the mounting table mounting the object: 100 mm/sec

FIG. 12 is a view showing a photograph of a cross section of a part of asilicon wafer cut by laser processing under the conditions mentionedabove. A molten processed region 13 is formed within the silicon wafer11. The molten processed region 13 formed under the above-mentionedconditions has a size of about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 13 is a graph showingrelationships between the laser light wavelength and the transmittancewithin the silicon substrate. Here, the respective reflected componentson the front and rear sides of the silicon substrate are eliminated, soas to show the internal transmittance alone. The respectiverelationships are shown in the cases where the thickness t of thesilicon substrate is 50 μm, 100 μm, 200 μm, 500 μm, and 1000 μm.

For example, at the Nd:YAG laser wavelength of 1064 nm, the laser lightappears to be transmitted through the silicon substrate by at least 80%when the silicon substrate has a thickness of 500 μm or less. Since thesilicon wafer 11 shown in FIG. 12 has a thickness of 350 μm, the moltenprocessed region 13 caused by multiphoton absorption is formed near thecenter of the silicon wafer 11, i.e., at a part distanced from the frontface by 175 μm. The transmittance in this case is 90% or more withreference to a silicon wafer having a thickness of 200 μm, whereby thelaser light is absorbed only slightly within the silicon wafer 11 but issubstantially transmitted therethrough. This means that the moltenprocessed region 13 is formed within the silicon wafer 11 not by laserlight absorption within the silicon wafer 11 (i.e., not by usual heatingwith the laser light) but by multiphoton absorption. The forming of amolten processed region by multiphoton absorption is disclosed, forexample, in “Silicon Processing Characteristic Evaluation by PicosecondPulse Laser”, Preprints of the National Meetings of Japan WeldingSociety, Vol. 66 (April, 2000), pp. 72-73.

A fracture is generated in a silicon wafer from a cutting start regionformed by a molten processed region, acting as a start point, toward across section, and reaches the front and rear faces of the siliconwafer, whereby the silicon wafer is cut. The fracture reaching the frontand rear faces of the silicon wafer may grow naturally or as a force isapplied to the object. The fracture naturally growing from the cuttingstart region to the front and rear faces of the wafer encompasses a casewhere the fracture grows from a state where the molten processed regionforming the cutting start region is molten and a case where the fracturegrows when the molten processed region forming the cutting start regionis re-solidified from the molten state. In either case, the moltenprocessed region is formed only within the wafer, and thus is presentonly within the cross section after cutting as shown in FIG. 12. Whenthe molten processed region is formed within the object, unnecessaryfractures deviating from a line to cut are harder to occur at the timeof cleaving, whereby cleavage control becomes easier.

(3) Case where the Modified Region is a Refractive Index Change Region

An object to be processed (e.g., glass) is irradiated with laser lightwhile locating a light-converging point within the object under acondition with a field intensity of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 ns or less. Whenmultiphoton absorption is generated within the object with a very shortpulse width, the energy caused by multiphoton absorption is notconverted into thermal energy, whereby an eternal structure change suchas ion valence change, crystallization, or orientation polarization isinduced within the object, thus forming a refractive index changeregion. The upper limit of field intensity is 1×10¹² (W/cm²), forexample. The pulse width is preferably 1 ns or less, for example, morepreferably 1 ps or less. The forming of a refractive index change regionby multiphoton absorption is disclosed, for example, in “Forming ofPhotoinduced Structure within Glass by Femtosecond Laser Irradiation”,Proceedings of the 42nd Laser Materials Processing Conference (November1997), pp. 105-111.

In the above, cases (1) to (3) are explained as a modified region formeddue to multiphoton absorption within the object, but when under theconsideration of crystal structure and cleave of a wafer type object tobe processed, cutting start region is formed as stated below, it ispossible to cut a object to be processed with smaller power and higheraccuracy by using the cutting start region as a start point of theobject.

That is, in the case that the object to be processed is a substrate madeof a single crystal semiconductor having a diamond structure such as asilicon, it is preferable to form a cutting start region in a directionalong (1, 1, 1) (the first cleavage plane) or (1, 1, 0) plane (thesecond cleavage plane) of the object in the object. Further, in the casethat the object to be processed is a substrate made of III-V groupcompound semiconductor having a blende type crystal structure such as aGaAs, it is preferable to form a cutting start region in a directionalong (1, 1, 0) plane of the object in the object. More further, in thecase that the object to be processed is a substrate having a hexagonaltype crystal structure such as Safire (Al₂O₃), it is preferable to forma cutting start region in a direction along (1, 1, 2, 0) plane (A plane)or (1, 1, 0, 0) plane (M plane) of the object in the object when a mainsurface of the object is (0, 0, 0, 1) plane (C plane).

Besides, when an orientation flat is formed in the substrate along theabove stated direction along which the above explained cutting startregion should be formed in the object, for example a direction along (1,1, 1) plane in the single crystal silicon substrate) or along adirection perpendicular to the direction along which the above explainedcutting start region should be formed in the object, it is possible toform the cutting start region along the direction along which thecutting start region should be formed in the object in the object witheasy and high accuracy by using the orientation flat as a reference.

The invention will be explained concretely, referring embodimentshereinbelow.

EXAMPLE 1

Example 1 of the present invention will now be explained. The laserprocessing method in accordance with Example 1 comprises a modifiedregion forming step (first step) of forming a modified region due tomultiphoton absorption within an object to be processed, and a stressstep (second step) of generating a stress in a portion to cut theobject. In Example 1, the modified region forming step and stress stepperform the same laser light irradiation. Therefore, a laser processingapparatus which will be explained later emits laser light twice underthe same condition in the modified region forming step and stress step.

The laser processing apparatus in accordance with Example 1 will now beexplained. FIG. 14 is a schematic diagram of a laser processingapparatus 100 used in the modified region forming step. As depicted, thelaser processing apparatus 100 comprises a laser light source 101 forgenerating laser light L; a laser light source controller 102 forcontrolling the laser light source 101 in order to regulate the output,pulse width, and the like of the laser light L; a dichroic mirror 103arranged so as to change the orientation of the optical axis of thelaser light L by 90° while functioning to reflect the laser light L; acondenser lens 105 for converging the laser light L reflected by thedichroic mirror 103; a mount table 107 for mounting an object to beprocessed 1 to be irradiated with the laser light L converged by thecondenser lens 105; an X-axis stage 109 for moving the mount table 107along an X axis; a Y-axis stage 111 for moving the mount table 107 alonga Y axis which is orthogonal to the X axis; a Z-axis stage 113 formoving the mount table 107 along a Z-axis which is orthogonal to X and Yaxes; and a stage controller 115 for controlling the movement of thethree stages 109, 111, 113. In Example 1, the object 1 is a Pyrex(registered trademark) glass wafer.

The Z axis is orthogonal to the front face 3 of the object 1, and thusis the direction of focal depth of the laser light incident on theobject 1. Therefore, the light-converging point P of the laser light Lcan be positioned within the object 1 by moving the Z-axis stage 113along the Z axis. The movement of the light-converging point P along theX (Y) axis is performed by moving the object 1 along the X (Y) axis bythe X (Y)-axis stage 109 (111).

The laser light source 101 is Nd:YAG laser generating pulsed laserlight. Other examples of the laser employable in the laser light source101 include Nd:YVO₄ laser and Nd:YLF laser. The above-mentioned laserlight sources are preferably used for forming a crack region or a moltenprocessed region, whereas a titanium sapphire laser is preferably usedfor forming a refractive index change region. Though Example 1 usespulsed laser light for processing the object 1, continuous wave laserlight may also be used if it can cause multiphoton absorption.

The laser processing apparatus 100 further comprises an observationlight source 117 for generating visible rays for illuminating the object1 mounted on the mount table 107, and a visible ray beam splitter 119disposed on the same optical axis as with the dichroic mirror 103 andcondenser lens 105. The dichroic mirror 103 is disposed between the beamsplitter 119 and condenser lens 105. The beam splitter 119 functions toreflect about a half of the visible rays and transmit the remaining halftherethrough, and is disposed so as to change the orientation of theoptical axis of visible rays by 90°. About a half of the visible raysgenerated from the observation light source 117 are reflected by thebeam splitter 119. Thus reflected visible rays pass through the dichroicmirror 103 and condenser lens 105, thereby illuminating the front face 3of the object 1 including the line 5 along which the object is intendedto be cut and the like.

The laser processing apparatus 100 further comprises an image pickupdevice 121 and an imaging lens 123 which are disposed on the sameoptical axis as with the beam splitter 119, dichroic mirror 103, andcondenser lens 105. An example of the image pickup device 121 is a CCD(charge-coupled device) camera. The reflected light of visible rayshaving illuminated the front face 3 including the line 5 along which theobject is intended to be cut and the like passes through the condenserlens 105, dichroic mirror 103, and beam splitter 119, so as to befocused by the imaging lens 123 and captured by the image pickup device121, thus yielding imaging data.

The laser processing apparatus 100 further comprises an imaging dataprocessor 125 for inputting the imaging data outputted from the imagepickup device 121, an overall controller 127 for controlling the laserprocessing apparatus 100 as a whole, and a monitor 129. Based on theimaging data, the imaging data processor 125 calculates focal data forpositioning the focal point of visible rays generated by the observationlight source 117 onto the front face 3. According to the focal data, thestage controller 115 regulates the movement of the Z-axis stage 113, soas to position the focal point of visible rays at the front face 3.Thus, the imaging data processor 125 functions as an autofocus unit. Thefocal point of visible light coincides with the light-converging pointof laser light L. On the basis of imaging data, the imaging dataprocessor 125 calculates image data such as enlarged images of the frontface 3. The image data are sent to the overall controller 127, so as tobe subjected to various processing operations, and thus processed dataare transmitted to the monitor 129. As a consequence, enlarged imagesand the like are displayed on the monitor 129.

The data from the stage controller 115, the image data from the imagingdata processor 125, etc. are fed into the overall controller 127,whereas the laser light source controller 102, observation light source117, and stage controller 115 are regulated according to these data aswell, whereby the laser processing apparatus 100 as a whole iscontrolled. Hence, the overall controller 127 functions as a computerunit.

With reference to FIGS. 14 and 15, the laser processing method inaccordance with Example 1 will now be explained. FIG. 15 is a flowchartfor explaining the laser processing method.

First, the light absorption characteristic of the object 1 is measuredby a spectrophotometer or the like which is not depicted. According tothe result of measurement, a laser light source 101 which generateslaser light L having a wavelength to which the object 1 is transparentor less absorptive is chosen (S101). Subsequently, the thickness of theobject 1 is measured. According to the result of measurement ofthickness and the refractive index of the object 1, the amount ofmovement of the object 1 along the Z axis in the laser processingapparatus 100 is determined (S103). This is an amount of movement of theobject 1 along the Z axis with reference to the light-converging point Pof laser light L positioned at the front face 3 of the object 1 forlocating the light-converging point P of laser light L within the object1. This amount of movement is fed into the overall controller 127 in thelaser processing apparatus 100 used in the modified region forming step.

The object 1 is mounted on the mount table 107 of the laser processingapparatus 100. Then, visible rays are generated from the observationlight source 117, so as to illuminate the object 1 (S105). The frontface 3 of the object 1 including the illuminated line 5 along which theobject is intended to be cut is captured by the image pickup device 121.The imaging data captured by the image pickup device 121 is sent to theimaging data processor 125. According to the imaging data, the imagingdata processor 125 calculates such focal data as to position the focalpoint of visible rays from the observation light source 117 onto thefront face 3 (S107).

The focal data is sent to the stage controller 115. According to thefocal data, the stage controller 115 moves the Z-axis stage 113 alongthe Z axis (S109). As a consequence, the focal point of the visible raysfrom the observation light source 117 is positioned at the front face 3.According to the imaging data, the imaging data processor 125 calculatesenlarged image data of the front face 3 of the object 1 including theline 5 along which the object is intended to be cut. The enlarged imagedata is sent to the monitor 129 by way of the total controller 127,whereby an enlarged image of the line 5 along which the object isintended to be cut and its vicinity is displayed on the monitor 129.

The movement amount data determined by step S103 has been fed into thetotal controller 127 beforehand, and is sent to the stage controller115. According to the movement amount data, the stage controller 115causes the Z-axis stage 113 to move the object 1 along the Z axis tosuch a position that the light-converging point P of laser light L islocated within the object 1 (S111).

Subsequently, laser light L is generated from the laser light source101, so as to illuminate the line 5 along which the object is intendedto be cut in the front face 3 of the object 1. FIG. 16 is a sectionalview of the object 1 including a crack region 9 during the laserprocessing in the modified region forming step. Since thelight-converging point P of laser light L is located within the object 1as depicted, the crack region 9 is formed only within the object 1.Then, the X-axis stage 109 and Y-axis stage 111 are moved along the line5 along which the object is intended to be cut, so as to form the crackregion 9 within the object 1 along the line 5 along which the object isintended to be cut (S113).

After the modified region is formed, the crack region 9 is irradiatedwith the laser light L along the line 5 along which the object isintended to be cut in the front face 3 of the object 1 again under thesame condition (i.e., while the light-converging point P is located atthe crack region 9 that is a modified region). As a consequence, theabsorption of laser light L due to scattering and the like by the crackregion 9 or the occurrence of multiphoton absorption in the crack regionheats the object 1 along the crack region 9, thereby causing a stresssuch as thermal stress due to a temperature difference (S114). FIG. 17is a sectional view of the object 1 including the crack region 9 duringthe laser processing in the stress step. As depicted, cracks furthergrow and reach the front face 3 and rear face 21 of the object 1 fromthe crack region 9 acting as a start point, so as to form a cut section10 in the object 1, thereby cutting the object 1 (S115). This dividesthe object 1 into silicon chips.

Though the stress step performs the same laser light irradiation as withthe modified region in Example 1, it will be sufficient if laser lighttransmittable through the unmodified region that is the region notformed with the crack region in the object and more absorbable by thecrack region than by the unmodified region is emitted. This is becausethe laser light is hardly absorbed at the front face of the object inthis case as well, so that the object is heated along the crack region,whereby a stress such as thermal stress due to a temperature differenceoccurs.

Though Example 1 relates to a case where a crack region is formed as amodified region, the same holds in cases where the above-mentionedmolten processed region and refractive index change region are formed.That is, irradiation with absorbable laser light can cause a stress, soas to generate and grow cracks from the molten processed region orrefractive index change region acting as a start point, thereby cuttingthe object to be processed.

Even when cracks grown from the modified region acting as a start pointby the stress step fail to reach the front and rear faces of the objectin the case where the object has a large thickness, etc., the object canbe fractured and cut by applying an artificial force such as bendingstress or shear stress thereto. This artificial force can be keptsmaller, whereby unnecessary fractures deviating from the line to cutcan be prevented from occurring in the front face of the object.

Effects of Example 1 will now be explained. In this case, the line 5along which the object is intended to be cut is irradiated with pulsedlaser light L while locating a light-converging point P within theobject 1 under a condition generating multiphoton absorption in themodified region forming step. Then, the X-axis stage 109 and Y-axisstage 111 are moved, so as to shift the light-converging point P alongthe line 5 along which the object is intended to be cut. This forms amodified region (e.g., crack region, molten processed region, orrefractive index change region) within the object 1 along the line 5along which the object is intended to be cut. When a start point existsin a portion to cut the object, the object can be fractured and cut witha relatively small force. In Example 1, the stress step performs thesame laser irradiation as with the modified region forming step, therebycausing a stress such as thermal stress due to a temperature difference.As a consequence, a relatively small force typified by a stress such asthermal stress due to a temperature difference can cut the object 1.This can cut the object 1 without generating unnecessary fracturesdeviating from the line 5 along which the object is intended to be cutin the front face 3 of the object 1.

In the modified region forming step in Example 1, since the object 1 isirradiated with pulsed laser light L while locating the light-convergingpoint P within the object 1 under a condition generating multiphotonabsorption in the object 1, the pulsed laser light L is transmittedthrough the object 1, whereby the pulsed laser light is hardly absorbedby the front face 3 of the object 1. The stress step performs the samelaser light irradiation as with the modified region forming step.Therefore, the front face 3 does not incur damages such as meltingbecause of irradiation with laser light.

As explained in the foregoing, Example 1 can cut the object 1 withoutgenerating unnecessary fractures deviating from the line 5 along whichthe object is intended to be cut in the front face 3 of the object 1 ormelting the same. Therefore, when the object 1 is a semiconductor wafer,for example, semiconductor chips can be cut out from the semiconductorwafer without generating unnecessary fractures deviating from a line tocut in the semiconductor chips or melting the same. The same holds inobjects to be processed having a front face formed with electronicdevices such as objects to be processed having a front face formed withelectrode patterns, piezoelectric device wafers, and glass substratesformed with display devices such as liquid crystals. Hence, Example 1can improve the yield of products (e.g., semiconductor chips,piezoelectric device chips, and display devices such as liquid crystals)made by cutting objects to be processed.

In Example 1, since the line 5 along which the object is intended to becut in the front face 3 of the object 1 does not melt, the width of theline 5 along which the object is intended to be cut (which is the gapbetween respective regions to become semiconductor chips in the case ofa semiconductor wafer, for example) can be made smaller. This increasesthe number of products formed from a single sheet of object to beprocessed 1, and can improve the productivity of products.

Example 1 uses laser light for cutting and processing the object 1, andthus enables processing more complicated than that in dicing withdiamond cutters. For example, cutting and processing is possible evenwhen lines 5 along which the object is intended to be cut havecomplicated forms as shown in FIG. 18.

EXAMPLE 2

Example 2 of the present invention will now be explained. FIGS. 20 to 23are partial sectional views of the object 1 taken along the line XX-XXof FIG. 19.

As shown in FIGS. 19 and 20, an expandable film (holding member) 19 isattached to the rear face 21 of the object 1, whereas the object 1 issecured onto the front face 19 a of the expandable film 19. Theexpandable film 19 has an outer peripheral part attached to aring-shaped film securing frame 20, so as to be secured thereto. Theobject 1 is a silicon wafer having a thickness of 100 μm.

A unit U thus constructed by the object 1, expandable film 19, and filmsecuring frame 20 is mounted on the mount table 107 of theabove-mentioned laser processing apparatus 100, for example, such thatthe front face 3 of the object 1 opposes the condenser lens 105. Then, aholder 107 a secures the film securing frame 20 onto the mount table107, while the expandable film 19 is attracted to the mount table 107 ina vacuum.

Subsequently, as shown in FIG. 19, lines 5 along which the object isintended to be cut extending in directions parallel and perpendicular toan orientation flat 16 of the object 1 are set like a grid. The linesalong which the object is intended to be cut are set between deviceforming surfaces 700 each made of a functional device such as circuitdevice or light-receiving surface formed on the wafer. For simplicity,the device forming surfaces 700 are illustrated only partly in thedrawing.

Then, as shown in FIG. 20, the object 1 is irradiated with laser lightL1 while locating a light-converging point P1 within the object 1, andthe light-converging point P1 is moved along the lines 5 along which theobject is intended to be cut, so as to form a modified region 7 withinthe object 1. This modified region 7 forms a cutting start region 8inside of the front face (laser light entrance surface) 3 of the object1 by a predetermined distance along the lines 5 along which the objectis intended to be cut. Since the object 1 is a silicon wafer, a moltenprocessed region is formed as the modified region 7.

Subsequently, as shown in FIG. 21, the object 1 is irradiated with laserlight L2 transmittable through the unmodified region of the object 1(the part other than the modified region 7 in the object 1) andpreferably more absorbable by the modified region 7 than by theunmodified region while locating a light-converging point P2 within themodified region 7 of the object 1, and the light-converging point P2 ismoved along the lines 5 along which the object is intended to be cut.The irradiation with the laser light L2 generates fractures 24 from thecutting start region 8 acting as a start point, whereby the fractures 24reach the front face 3 and rear face 21 of the object 1. As aconsequence, the object 1 is divided into a plurality of chips 25 alongthe lines 5 along which the object is intended to be cut. In Example 2,YAG laser having a wavelength of 1064 nm is used as the laser light L2.

A main cause of such fractures 24 is that the object 1 is heated alongthe lines 5 along which the object is intended to be cut uponirradiation with the laser light L2, whereby a thermal stress occurs inthe object 1. For example, upon irradiation with the laser light L2, aboundary between modified and unmodified regions yields continuousirregularities as shown in FIG. 12 instead of a smooth surface, so thatthe laser light L2 is scattered by the boundary between the modifiedregion 7 and the unmodified region, whereby the part surrounding themodified region 7 is heated. This heat generates fine cracks anddistortions in the boundary between the modified region 7 and theunmodified region, so as to cause a tensile stress starting from thesecracks and distortions, whereby fractures 24 occur from the modifiedregion 7 to the front face 3 or rear face 21.

After the object 1 is cut into a plurality of chips 25, the unit U istransferred to a film expander 200. As shown in FIG. 22, the unit U hasits film securing frame 20 held between a ring-shaped receptacle 201 anda ring-shaped holder 202, so as to be secured to the film expander 200.Then, a cylindrical pressing member 203 disposed on the inside of thereceptacle 201 is pressed against the rear face 19 b of the expandablefilm 19 from the lower side of the unit U, and is raised as shown inFIG. 23. This expands contact portions of the individual chips 25 in theexpandable film 19 outward, so as to separate the chips 25 from eachother, whereby the chips 25 can be picked up easily and reliably.

In the foregoing laser processing method in accordance with Example 2,the modified region 7 formed by multiphoton absorption can form thecutting start region 8 within the object 1 along the lines 5 along whichthe object is intended to be cut. Irradiating the object 1 with laserlight L2 transmittable through the unmodified region of the object 1(preferably more absorbable by the modified region 7 than by theunmodified region) along the lines 5 along which the object is intendedto be cut can generate fractures 24 in the object 1 from the cuttingstart region 8 acting as a start point, whereby the object 1 can be cutaccurately along the lines 5 along which the object is intended to becut. Expanding the expandable film 19 having the object 1 securedthereto separates the chips 25 from each other, which can furtherimprove the reliability in cutting the object 1 along the line 5 alongwhich the object is intended to be cut.

EXAMPLE 3

Example 3 of the present invention will now be explained. Example 3differs from Example 2 in that fractures 24 do not reach the front face3 and rear face 21 of the object 1. In the following, the differencesfrom Example 2 will mainly be explained. FIG. 24 is a partial sectionalview of the object 1 taken along the line XX-XX of FIG. 19.

As in Example 2, a unit U constituted by the object 1, an expandablefilm 19, and a film securing frame 20 is prepared, a modified region 7is formed within the object 1 by using the above-mentioned laserprocessing apparatus 100, and a cutting start region 8 is formed by themodified region 7 inside of the front face 3 of the object 1 by apredetermined distance along lines 5 along which the object is intendedto be cut. The object 1 is a silicon wafer having a thickness of 300 μm.

Subsequently, as shown in FIG. 24, the object 1 is irradiated with laserlight L2 transmittable through the unmodified region of the object 1 andpreferably more absorbable by the modified region 7 than by theunmodified region while locating a light-converging point P2 within themodified region 7 of the object 1, and the light-converging point P2 ismoved along the lines 5 along which the object is intended to be cut.The irradiation with the laser light L2 generates fractures 24 from thecutting start region 8 acting as a start point. Since the thickness (300μm) of the object 1 in Example 3 is greater than the thickness (100 μm)of the object 1 in Example 2, the fractures 24 stay therewithin withoutreaching the front face 3 and rear face 21 of the object 1. Theirradiation condition of laser light L2 is the same as that in Example2.

Next, as in Example 2, the unit U is transferred to the film expander200. In the film expander 200, the pressing member 203 is pressedagainst the rear face 19 b of the expandable film 19, and is raised.This expands contact portions of the individual chips 25 in theexpandable film 19 outward. As the expandable film 19 expands, leadingends of the fractures 24 within the object 1 reach the front face 3 andrear face 21 of the object 1, so that the object 1 is divided into aplurality of chips 25, whereby the chips 25 are separated from eachother.

Depending on the irradiation condition of laser light L2, the fractures24 may not occur upon irradiation with the laser light L2. Even in sucha case, expanding the expandable film 19 can cut the object 1 along thelines 5 along which the object is intended to be cut more easily with ahigher accuracy than in the case without irradiation with laser lightL2.

The foregoing laser processing method in accordance with Example 3 canform the cutting start region 8 within the object 1 along the lines 5along which the object is intended to be cut as with the above-mentionedlaser processing method in accordance with Example 2. Then, irradiatingthe object 1 with laser light L2 transmittable through the unmodifiedregion of the object 1 (preferably more absorbable by the modifiedregion 7 than by the unmodified region) along the lines 5 along whichthe object is intended to be cut can cause the fractures 24 started fromthe cutting start region 8 to reach the front face 3 and rear face 21 ofthe object 1 with a force smaller than that in the case without suchirradiation. Therefore, the expandable film 19 having the object 1secured thereto can be expanded with a smaller force, and the object 1can be cut accurately along the lines 5 along which the object isintended to be cut. Expanding the expandable film 19 separates the chips25 from each other, whereby the reliability in cutting the object 1along the lines 5 along which the object is intended to be cut canfurther be improved.

The present invention is not restricted to Examples 1 to 3 mentionedabove.

The following are preferred examples of the material of the object 1 andspecies of laser light L2 transmittable through the unmodified region ofthe object 1 and more absorbable by the modified region 7 than by theunmodified region. Namely, when the object 1 is a silicon wafer orGaAs-based wafer, laser light having a wavelength of 900 nm to 1100 nmis preferably used as the laser light L2. A specific example is YAGlaser (with a wavelength of 1064 nm).

The fractures 24 generated upon irradiation with the laser light L2 mayreach one of the front face 3 and rear face 21 of the object 1. Suchcontrol is possible when the modified region 7 is formed at a positionshifted from the center position in the thickness direction of theobject 1 toward the front face 3 or rear face 21. In particular, whenthe fractures 24 are caused to reach the surface of the object 1 on theexpandable film 19 side upon irradiation with the laser light L2, theaccuracy in cleaving the object 1 by expanding the expandable film 19can further be improved.

Here, “the modified region 7 is formed at a position shifted from thecenter position in the thickness direction of the object 1 toward thefront face 3” means that the modified region 7 constituting the cuttingstart region 8 is formed so as to shift from the half thickness positionof the object 1 in the thickness direction toward the front face 3.Namely, it refers to a case where the center position of the width ofthe modified region in the thickness direction of the object 1 isshifted from the center position of the object 1 in the thicknessdirection toward the front face 3, without being restricted to the casewhere the whole modified region 7 is shifted from the center position ofthe object 1 in the thickness direction toward the front face 3. Thesame holds in the case where the modified region 7 is formed so as toshift toward the rear face 21 of the object 1.

Though the above-mentioned light-converging point P2 of the laser lightL2 is positioned within the modified region 7 of the object 1, it may bepositioned in the vicinity of the modified region 7 as long as themodified region 7 is irradiated with the laser light L2.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, the laser processing method in accordancewith the present invention can cut the object to be processed accuratelyalong lines along which the object is intended to be cut.

1. A laser processing method comprising: a first step of irradiating awafer-like object to be processed with laser light while locating alight-converging point within the object, so as to form a modifiedregion due to multiphoton absorption within the object, and causing themodified region to form a cutting start region inside of a laser lightentrance surface of the object by a predetermined distance along a lineto cut in the object; and a second step of irradiating the modifiedregion with laser light transmittable through an unmodified region ofthe object and more absorbable by the modified region than by theunmodified region after the first step, so as to generate a stress at aportion where the object is cut along the line to cut.
 2. A laserprocessing method comprising: a first step of irradiating a wafer-likeobject to be processed with laser light while locating alight-converging point within the object under a condition with a peakpower density of at least 1×10⁸ (W/cm²) at the light-converging pointand a pulse width of 1 μs or less, so as to form a modified regionincluding a crack region within the object, and causing the modifiedregion to form a cutting start region inside of a laser light entrancesurface of the object by a predetermined distance along a line to cut inthe object; and a second step of irradiating the modified region withlaser light transmittable through an unmodified region of the object andmore absorbable by the modified region than by the unmodified regionafter the first step, so as to generate a stress at a portion where theobject is cut along the line to cut.
 3. A laser processing methodcomprising: a first step of irradiating a wafer-like object to beprocessed with laser light while locating a light-converging pointwithin the object under a condition with a peak power density of atleast 1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1μs or less, so as to form a modified region including a molten processedregion within the object, and causing the modified region to form acutting start region inside of a laser light entrance surface of theobject by a predetermined distance along a line to cut in the object;and a second step of irradiating the modified region with laser lighttransmittable through an unmodified region of the object and moreabsorbable by the modified region than by the unmodified region afterthe first step, so as to generate a stress at a portion where the objectis cut along the line to cut.
 4. A laser processing method comprising: afirst step of irradiating a wafer-like object to be processed with laserlight while locating a light-converging point within the object under acondition with a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 ns or less, so as to forma modified region including a refractive index change region as a regionwith a changed refractive index within the object, and causing themodified region to form a cutting start region inside of a laser lightentrance surface of the object by a predetermined distance along a lineto cut in the object; and a second step of irradiating the modifiedregion with laser light transmittable through an unmodified region ofthe object and more absorbable by the modified region than by theunmodified region after the first step, so as to generate a stress at aportion where the object is cut along the line to cut.
 5. A laserprocessing method according to one of claims 1 to 4, wherein the secondstep performs the same laser light irradiation as with the first stepwhile locating a light-converging point at the modified region.
 6. Alaser processing method comprising the steps of: irradiating awafer-like object to be processed secured to a surface of an expandableholding member with laser light while locating a light-converging pointwithin the object, so as to form a modified region within the object,and causing the modified region to form a cutting start region inside ofa laser light entrance surface of the object by a predetermined distancealong a line to cut in the object; irradiating the modified region withlaser light transmittable through an unmodified region of the objectafter the step of forming the cutting start region, so as to cut theobject along the line to cut; and expanding the holding member after thestep of cutting the object, so as to separate cut portions of the objectfrom each other.
 7. A laser processing method comprising the steps of:irradiating a wafer-like object to be processed secured to a surface ofan expandable holding member with laser light while locating alight-converging point within the object, so as to form a modifiedregion within the object, and causing the modified region to form acutting start region inside of a laser light entrance surface of theobject by a predetermined distance along a line to cut in the object;irradiating the modified region with laser light transmittable throughan unmodified region of the object after the step of forming the cuttingstart region; and expanding the holding member after the step ofirradiating the object, so as to cut the object and separate cutportions of the object from each other.
 8. A laser processing methodaccording to claim 6 or 7, wherein the object is formed from asemiconductor material, and wherein the modified region is a moltenprocessed region.
 9. A laser processing method comprising the steps of:irradiating a wafer-like object to be processed made of a semiconductormaterial with laser light while locating a light-converging point withinthe object, so as to form a molten processed region within the object,and causing the molten processed region to form a cutting start regioninside of a laser light entrance surface of the object by apredetermined distance; and irradiating the object with laser lighttransmittable through an unmodified region of the object after the stepof forming the cutting start region.
 10. A laser processing methodcomprising; a first step of irradiating a wafer-like object to beprocessed with laser light while locating a light-converging pointwithin the object, so as to form a modified region due to multiphotonabsorption within the object, and causing the modified region to form acutting start region inside of a laser light entrance surface of theobject by a predetermined distance along a line along which the objectis intended to be cut in the object; and a second step of irradiatingthe modified region with laser light transmittable through an unmodifiedregion of the object and more absorbable by the modified region than bythe unmodified region after the first step, so as to change the cuttingstart region into one easier to generate a fracture than is the cuttingstart region.
 11. A laser processing method comprising: a first step ofirradiating a wafer-like object to be processed with laser light whilelocating a light-converging point within the object under a conditionwith a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 μs or less, so as to forma modified region including a crack region within the object, andcausing the modified region to form a cutting start region inside of alaser light entrance surface of the object by a predetermined distancealong a line along which the object is intended to be cut in the object;and a second step of irradiating the modified region with laser lighttransmittable through an unmodified region of the object and moreabsorbable by the modified region than by the unmodified region afterthe first step, so as to change the cutting start region into one easierto generate a fracture than is the cutting start region.
 12. A laserprocessing method comprising: a first step of irradiating a wafer-likeobject to be processed with laser light while locating alight-converging point within the object under a condition with a peakpower density of at least 1×10⁸ (W/cm²) at the light-converging pointand a pulse width of 1 μs or less, so as to form a modified regionincluding a molten processed region within the object, and causing themodified region to form a cutting start region inside of a laser lightentrance surface of the object by a predetermined distance along a linealong which the object is intended to be cut in the object; and a secondstep of irradiating the modified region with laser light transmittablethrough an unmodified region of the object and more absorbable by themodified region than by the unmodified region after the first step, soas to change the cutting start region into one easier to generate afracture than is the cutting start region.
 13. A laser processing methodcomprising: a first step of irradiating a wafer-like object to beprocessed with laser light while locating a light-converging pointwithin the object under a condition with a peak power density of atleast 1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1ns or less, so as to form a modified region including a refractive indexchange region as a region with a changed refractive index within theobject, and causing the modified region to form a cutting start regioninside of a laser light entrance surface of the object by apredetermined distance along a line along which the object is intendedto be cut in the object; and a second step of irradiating the modifiedregion with laser light transmittable through an unmodified region ofthe object and more absorbable by the modified region than by theunmodified region after the first step, so as to change the cuttingstart region into one easier to generate a fracture than is the cuttingstart region.
 14. A laser processing method according to one of claims10 to 13, wherein the second step performs the same laser lightirradiation as with the first step while locating a light-convergingpoint at the modified region.
 15. A laser processing method according toone of claims 10 to 13, wherein the laser light irradiated in the firststep and the laser light irradiated in the second step are generated bya same laser light source.
 16. A laser processing method according toone of claims 10 to 13, wherein the laser light irradiated in the firststep and the laser light irradiated in the second step are pulsed laserlight, and are generated by a same laser light source.
 17. A laserprocessing method comprising the steps of: irradiating a wafer-likeobject to be processed secured to a surface of an expandable holdingmember with laser light while locating a light-converging point withinthe object, so as to form a modified region within the object, andcausing the modified region to form a cutting start region inside of alaser light entrance surface of the object by a predetermined distancealong a line along which the object is intended to be cut in the object;irradiating the modified region with the laser light transmittablethrough an unmodified region of the object and more absorbable by themodified region than by the unmodified region after the step of formingthe cutting start region, so as to change the cutting start region intoone easier to generate a fracture than is the cutting start region, andcutting the object along the line along which the object is intended tobe cut; and expanding the holding member after the step of cutting theobject, so as to separate cut portions of the object from each other.18. A laser processing method according to claim 17, wherein the step ofcutting the object performs the same laser light irradiation as with thestep of forming the cutting start region while locating alight-converging point at the modified region.
 19. A laser processingmethod according to claim 17, wherein the laser light irradiated in thestep of forming the cutting start region and the laser light irradiatedin the step of cutting the object are generated by a same laser lightsource.
 20. A laser processing method according to claim 17, wherein thelaser light irradiated in the step of forming the cutting start regionand the laser light irradiated in the step of cutting the object arepulsed laser light, and are generated by the same laser light source.21. A laser processing method according to claim 17, wherein the objectis formed from a semiconductor material, and wherein the modified regionis a molten processed region.
 22. A laser processing method comprisingthe steps of: irradiating a wafer-like object to be processed secured toa surface of an expandable holding member with laser light whilelocating a light-converging point within the object, so as to form amodified region within the object, and causing the modified region toform a cutting start region inside of a laser light entrance surface ofthe object by a predetermined distance along a line along which theobject is intended to be cut in the object; irradiating the modifiedregion with laser light transmittable through an unmodified region ofthe object and more absorbable by the modified region than by theunmodified region after the step of forming and cutting start region, soas to change the cutting start region into one easier to generate afracture than is the cutting start region; and expanding the holdingmember after the step of changing the cutting start region, so as to cutthe object and separate cut portions of the object from each other. 23.A laser processing method according to claim 22, wherein the step ofchanging the cutting start region performs the same laser lightirradiation as with the step of forming the cutting start region whilelocating a light-converging point at the modified region.
 24. A laserprocessing method according to claim 22, wherein the laser lightirradiated in the step of forming the cutting start region and the laserlight irradiated in the step of changing the cutting start region aregenerated by a same laser light source.
 25. A laser processing methodaccording to claim 22, wherein the laser light irradiated in the step offorming the cutting start region and the laser light irradiated in thestep of changing the cutting start region are pulsed laser light, andare generated by a same laser light source.
 26. A laser processingmethod according to claim 22, wherein the object is formed from asemiconductor material, and wherein the modified region is a moltenprocessed region.
 27. A laser processing method comprising the steps of:irradiating a wafer-like object to be processed made of a semiconductormaterial with laser light while locating a light-converging point withinthe object, so as to form a molten processed region within the object,and causing the molten processed region to form a cutting start regioninside of a laser light entrance surface of the object by apredetermined distance; and irradiating the modified region with laserlight transmittable through an unmodified region of the object and moreabsorbable by the modified region than by the unmodified region afterthe step of forming the cutting start region, so as to change thecutting start region into one easier to generate a fracture than is thecutting start region.
 28. A laser processing method according to claim27, wherein the step of changing the cutting start region performs thesame laser light irradiation as with the step of forming the cuttingstart region while locating a light-converging point at the modifiedregion.
 29. A laser processing method according to claim 27, wherein thelaser light irradiated in the step of forming the cutting start regionand the laser light irradiated in the step of changing the cutting startregion are generated by a same laser light source.
 30. A laserprocessing method according to claim 27, wherein the laser lightirradiated in the step of forming the cutting start region and the laserlight irradiated in the step of changing the cutting start region arepulsed laser light, and are generated by a same laser light source.